The present invention relates in general to communications circuits and, more particularly, to modulator and demodulator circuits.
Wireless communications systems commonly use both an in-phase and a quadrature signal in modulating or demodulating information on a radio frequency carrier signal. In-phase and quadrature signals are used in such diverse applications as satellite receivers, cellular and cable telephone systems, global positioning systems and cable television set-top boxes. These applications operate at high frequencies between 500 megahertz and 3 gigahertz. A quadrature signal is one which is 90 degrees shifted in phase with respect to an in-phase signal.
Accurate modulation or demodulation depends on the in-phase signal and quadrature signals having a small quadrature phase error and a constant amplitude. These goals are difficult to achieve at higher frequencies, however, because circuit parasitics and signal propagation delays have a greater effect at high frequencies. The result is either a more complex circuit or lower performance of the modulator or demodulator.
A previously known method generates the quadrature signal passively by applying the in-phase signal to a resistor-capacitor network. The network shifts the phase of the in-phase signal by 90 degrees, thereby producing the quadrature signal. However, this method is inadequate for systems which are tuned over a frequency range because the amplitude of the quadrature signals varies severely over the range. Another passive method using a tank circuit reduces amplitude variations by tuning the tank circuit with variable reactance devices such as varactor diodes. However, varactor diodes require large tuning voltage variations which are difficult to provide on a low voltage integrated circuit.
A digital method uses an input signal at twice the frequency of the desired in-phase and quadrature signals. A divide-by-two frequency divider generates the in-phase and quadrature signals from the input signal. However, the high input frequency makes this method impractical for high frequency applications. Another disadvantage is that the resulting quadrature signal does not have a fifty percent duty cycle so that additional signal processing is needed to produce a quadrature signal having a fifty percent duty cycle.
There is a need for a circuit and a method for accurately generating a quadrature signal over a range of high frequencies which reduces the amplitude variation in the quadrature signal. It would be a benefit if the method and circuit produced a quadrature signal without the need for additional circuitry to provide a fifty percent duty cycle in the quadrature signal. It would be a further benefit if large bias voltages were not required for tuning variable reactance devices so that the quadrature signal could be integrated on a semiconductor die with a minimum of external circuitry.